Torque load transfer attachment hardware

ABSTRACT

A bushing for assembling an exhaust duct liner with an exhaust duct in a gas turbine engine comprises a body, a bushing opening and a first tab. The body is insertable into a duct opening in an exhaust duct. The bushing opening extends through the body to receive a shaft of a liner fastener. The first tab protrudes from the body to extend into an interior of the duct to prevent rotation of a head of the liner fastener.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority as a divisional application under 35U.S.C.§121 of earlier filed application Ser. No. 11/514,293 entitled“TORQUE LOAD TRANSFER ATTACHMENT HARDWARE” by Jorge I. Farah, et al. andfiled Aug. 31, 2006, which is a continuation-in-part of application Ser.No. 11/326,004 entitled “DAMPED COIL PIN FOR ATTACHMENT HANGER HINGE” byJorge I. Farah, et al. and filed Jan. 5, 2006.

STATEMENT OF GOVERNMENT INTEREST

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.N00019-02-C-3003 awarded by the United States Navy.

BACKGROUND

This invention relates generally to gas turbine engines and moreparticularly to exhaust duct liner attachment systems and methods. Ingas turbine engines, it is necessary to protect exhaust ducts with aninsulating shield in order to prevent heated core gases from damagingthe exhaust ducts. Typically, exhaust ducts are made from titanium ortitanium alloys and have temperature limits in the vicinity of 400° F.(204.4° C.). Exhausted core gases can reach temperatures upwards of3500° F. (1648.89° C.). It is, therefore, necessary to line exhaustducts with a material capable of withstanding the peak temperatures ofthe core gases and that prevents the exhaust duct from reaching itstemperature limitations.

For particular operations, particularly in military operations, it isdesirable to have aircraft with conventional take-off and landing (CTOL)capabilities, and short take-off, vertical landing (STOVL) capabilities.CTOL requires conventional thrusting of the aircraft in the horizontaldirection, while STOVL requires thrusting of the aircraft in verticaland intermediate directions. Some dual capability aircraft designs thusemploy variable direction exhaust ducts, such as three bearing swivelducts (3BSDs), for directing thrust produced by the exhaust nozzle inboth the horizontal and vertical directions. Variable direction exhaustducts typically comprise multiple co-axial exhaust duct segments havingangled junctions, whereby the segments can be rotated with respect toeach other to redirect the direction of thrust. The exhaust ductsegments interface through swivel bearing joints, which extendspartially into the assembled exhaust duct. This has the effect ofrestricting the diameter of the exhaust duct near the swivel bearingjoints. In order to properly pre-load the swivel bearings, it istypically necessary to assemble the exhaust duct segments beforeattaching exhaust duct liners to the exhaust duct segments. It is,therefore, necessary to have an exhaust duct liner suspension systemthat can be inserted past the swivel bearing joint in, and secured to,an already assembled exhaust duct. This typically requires insertion offasteners into the interior of the duct from the exterior, which canlead to difficulties in aligning fasteners with the suspension system,and dropping of fasteners into the exhaust duct, from which they are noteasily recovered. It would be desirable to simplify the installationprocess of such suspension systems to, for example, reduce installationtime and insure proper installation alignment.

SUMMARY

The present invention is directed to a bushing for assembling an exhaustduct liner with an exhaust duct in a gas turbine engine. The bushingcomprises a body, a bushing opening and a first tab. The body isinsertable into a duct opening in an exhaust duct. The bushing openingextends through the body to receive a shaft of a liner fastener. Thefirst tab protrudes from the body to extend into an interior of the ductto prevent rotation of a head of the liner fastener.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an aircraft having a STOVL capable gas turbine engineincluding a three bearing swivel duct.

FIG. 2A shows the three bearing swivel duct of FIG. 1 configured forconventional operation.

FIG. 2B shows the three bearing swivel duct of FIG. 1 configured forvertical landing or take-off operation.

FIG. 3 shows a cut-away portion of the three bearing swivel duct ofFIGS. 2A and 2B.

FIG. 4 shows an exploded view of a duct liner suspension system used inthe three bearing swivel duct of FIG. 3.

FIG. 5 shows a partially cut away sectional view of a duct linersuspension system as taken along section 5-5 of FIG. 3.

FIG. 6A shows a perspective view of a torque bushing of the duct linersuspension system of FIG. 4.

FIG. 6B shows a bottom view of the torque bushing of FIG. 6A.

FIG. 6C shows a side view of the torque bushing of FIG. 6A.

FIG. 7 shows a side view of a torque bushing having two torque tabsinserted into an exhaust duct.

DETAILED DESCRIPTION

FIG. 1 shows jet-powered aircraft 10 (shown in phantom) having shorttake-off vertical landing (STOVL) gas turbine engine 12. Engine 12includes multiple thrust producing and thrust directing elements thatenable aircraft 10 to take-off on a shortened runway and landvertically. Engine 12 includes lift fan 14, lift fan shaft 16, powerplant 18, control ducts 20A and 20B, three bearing swivel duct 22 andexhaust nozzle 24. Power plant 18 is the primary thrust-producingelement of engine 12 and is used to produce thrust in the x direction.Three bearing swivel duct (3BSD) 22 directs the thrust of power plant 18in the x direction when in configuration A (as shown by 3BSD 22 in solidlines). 3BSD 22 is adjustable to redirect the thrust of power plant 18in the y direction when in configuration B (as shown by 3BSD 22 indashed lines). 3BSD 22 is also used to produce thrust in intermediatedirections. Nozzle 24 increases and focuses the thrust produced by powerplant 18 and is secured to the tail end of 3BSD 22. 3BSD 22 is used inconfiguration A, for example, during traditional take off and flightoperations of aircraft 10 in the x direction. 3BSD 22 is positioned inintermediate directions, for example, to facilitate short take-offoperations. 3BSD 22 is positioned in configuration B, for example, toassist lift fan 14 in vertical landing operations. Lift fan 14 isselectively driven by power plant 18 through lift fan shaft 16, and isused to produce thrust in the y direction near the forward portion ofaircraft 10. With 3BSD 22 producing thrust near the aft portion ofaircraft 10, lift fan 14 and power plant 18 control the pitch ofaircraft 10. During vertical landing operations, control ducts 20A and20B redirect a portion of the thrust produced by power plant 18 in the ydirection underneath the wings, at a location away from the axis onwhich power plant 18 and lift fan 14 produce thrust in the y direction.Typically, control ducts 20A and 20B are selectively engaged to balancethe roll of aircraft 10 during vertical landing and take-off operations.

FIG. 2A shows three bearing swivel duct (3BSD) 22 of FIG. 1 inconfiguration A with nozzle 24 oriented along the x axis. FIG. 2B shows3BSD 22 in configuration B with nozzle 24 oriented approximately 105°from the x axis. 3BSD 22 is positioned between power plant 18 and nozzle24 of engine 12. 3BSD 22 comprises front duct 26A, intermediate duct26B, rear duct 26C, front liner 28A, intermediate liner 28B, rear liner28C, a plurality of suspension systems 30 and swivel bearings 32A-32C.

Front duct 26A is connected with power plant 18 along a vertical axisusing forward swivel bearing 32A. Swivel bearing 32A allows front duct26A to rotate approximately 360° with respect to power plant 18. Swivelbearing 32A is controlled by a central control system of aircraft 10that positions front duct 26A for each desired operational mode ofaircraft 10. Similarly, intermediate duct 26B is connected with frontduct 26A using intermediate swivel bearing 32B. Swivel bearing 32B iscentrally controlled and allows intermediate duct 26B to rotateapproximately 360° with respect to front duct 26A. The body ofintermediate duct 26B is angled at angle b and the aft edge of frontduct 26A is angled at angle a such that when they rotate with respect toeach other, the position of nozzle 24 rotates about the x-axis.Likewise, rear duct 26C is connected with intermediate duct 26B usingaft swivel bearing 32C. Swivel bearing 32C is centrally controlled andallows rear duct 26C to rotate approximately 360° with respect tointermediate duct 26B. With the body of intermediate duct 26B beingangled at angle b, the forward edge of rear duct 26C is angled at anglec such that when it rotates, the position of nozzle 24 rotates about thex-axis. Angles a, b and c are selected such that in configuration A 3BSDis generally horizontal, but can be pivoted to reposition nozzle 24.FIG. 2B shows front duct 26A rotated 180° with respect to power plant18, intermediate duct 26B rotated 180° with respect to front duct 26Aand rear duct 26C rotated 180° with respect to intermediate duct 26B.Thus, due to the angles at which front duct 26A, intermediate duct 26Band rear duct 26C are joined (angles a, b and c), 3BSD 22 is angleddownward a total of about 105° with respect to the x axis inconfiguration B. Nozzle 22 can also be oriented approximately 40° fromthe x-axis (e.g. for short take-off operation) by rotating front duct26A, intermediate duct 26B and rear duct 26C approximately 180° each.

Because of manufacturing and assembly reasons, such as those describedabove, exhaust liners 28A-28C must be assembled with exhaust ducts26A-26C after exhaust ducts 26A-26C have been assembled to each other.Therefore, front liner 28A, intermediate liner 28B and rear liner 28Care suspended from front duct 26A, intermediate duct 26B and rear duct26C, respectively, using a plurality of suspension systems 30 that canbe secured from the exterior of ducts 26A-26C. Suspension systems 30span the difference in diameters of ducts 26A-26C and liners 28A-28C,and can be tailored to specific lengths. Typically there are abouttwenty rows of suspension systems evenly distributed along thecircumference of each duct segment, with rows at wider parts of theducts having more suspension systems. To simplify and improve assemblyand disassembly of the exhaust liners and the exhaust ducts, suspensionsystems 30 include duct torque bushings for receiving the fasteners usedto join suspensions systems 30 with exhaust ducts 26A-26C.

FIG. 3 shows a cut-away portion of intermediate duct 26B andintermediate duct liner 28B connected by suspension systems 30.Suspension systems 30 include cold sheet bracket 34, hinge 36, pin 38,hanger 40, bushing 42, fastener 44, lock nut 46 and axial stiffener 48.Suspension systems 30 connect intermediate duct 26B with intermediateliner 28B.

Cold sheet bracket 34 is connected with intermediate duct liner 28B atcorrugation 50. Cold sheet bracket 34 is inserted through hole 51 induct liner 28B from the interior of duct liner 28B. Hinge 36 forms arotatable connection with bracket 34 utilizing pin 38. Pin 38 iscompression fit into a bore in cold sheet bracket 34 such that pin willnot rotate with respect to cold sheet bracket 34, but hinge 36 isrotatable about pin 38. Typically, each cold sheet bracket, hinge andpin are pre-assembled as a hinge assembly before each bracket 34 isjoined with liner 28B. Once the pre-assembled hinge assemblies arefastened to liner 28B, hangers 40 are joined with each hinge 36 from theexterior of liner 28B. Hangers 40 are connected to each other by axialstiffener 48, which, among other things, provides axial load sharingbetween hangers 40. Hangers 40, hinge 36 and axial stiffener 48 arefastened together with, for example threaded fasteners or rivets. Due tostiffener 48, hangers 40 are simultaneously rotatable downward intocorrugations 50. Hangers 40 are rotated away from stiffener 48, asindicated by arrow R, such that stiffener 48 does not prevent hangers 40from laying down into corrugations 50. Thus, suspension systems 30 areinsertable past swivel bearing joints 32A-32C during assembly of 3BSD22.

After exhaust ducts 26A-26C are assembled to each other and to aircraft10, intermediate liner 28B is inserted into exhaust duct 26B. Hanger 40is then rotated upward such that suspension system 30 can be joined withexhaust duct 26B with bushing 42, fastener 44 and lock nut 46. Bushing42 provides a mistake-proof method for joining fastener 44 with hanger40. For example, bushing 42 insures that fastener 44 is properly seatedwith hanger 40 and allows for improved torque transfer from fastener 44to duct 26B during assembly and disassembly of nut 46 and fastener 44.

FIG. 4 shows an exploded view of duct liner suspension system 30 used inthe three bearing swivel duct of FIG. 3. Suspension system 30 includescold sheet bracket 34, hinge 36, pin 38, hanger 40, bushing 42, fastener44 and lock nut 46. Fastener 44, nut 46 and bushing 42 link first end 48of hanger 40 with intermediate duct 26B, while cold sheet bracket 34,hinge 36 and pin 38 link second end 50 of hanger 40 with intermediateliner 28B. After cold sheet bracket 34 and hanger 40 are linked withliner 28B, and liner 28B has been inserted into duct 26B, hanger 40 isrotated to extend radially outward from liner 28B to meet an interiorfacing side of duct 26B. The position of hanger 40 is variable about pin38 via oval shaped bores in hinge 36 such that first end 48 of hanger 40can be rotated past duct 26B and then pulled flush with the interior ofduct 26B.

Intermediate duct 26B includes duct opening 52, and first end 48 ofhanger 40 includes hanger opening 54 and fastener flanges 56. When firstend 48 is placed in position for assembly, duct opening 52 and hangeropening 54 substantially align with each other such that they are ableto receive fastener 44. Fastener 44 includes shaft 58 and head 60. Inone embodiment, shaft 58 comprises threaded channels for receiving andengaging corresponding channels in lock nut 46. Head 60 comprises anoblong shape, such that its width w is longer than its length 1. In oneembodiment fastener 44 is a T-bolt. The profile of openings 52 and 54correspond to the profile, i.e. length and width, of head 60 such thatfastener 44 will pass through openings 52 and 54 only when the widthsand lengths of the three features are substantially aligned. In theembodiment shown, openings 52 and 54 are positioned such that the lengthof each hole is parallel to the centerline of engine 12, but in otherembodiments could be perpendicular to the centerline or have some otherconfiguration. Once fastener 44 is inserted into openings 52 and 54,head 60 is rotatable in opening 62 of hanger 40. For proper installationof suspension system 30, head 60 is rotated, preferably ninety degrees,such that head 60 engages flanges 56 and fastener 44 is prevented frompulling out of openings 52 and 54.

Bushing 42 is insertable into opening 52 such that hole 64 fits aroundshaft 58. Hole 64 provides a tight tolerance fit around shaft 58, andbushing 42 provides a tight tolerance fit in opening 52 such that duct26B is sealed by bushing 42. Additional washers can be included betweenduct 26B and nut 46, or duct 26B and head 60 to further enhance the sealonce assembled. Bushing 42 includes tab 66, which provides for properorientation of fastener head 60, and provides torque load distributionfor removing nut 46 from shaft 58 during disassembly of suspensionsystem 30.

FIG. 5 shows a partially cut away sectional view of duct linersuspension system 30 as taken along section 5-5 of FIG. 3. Suspensionsystem 30 is assembled with intermediate duct 26B such that bushing 42is properly seated in duct opening 52 and head 60 of fastener 44 isproperly aligned with hanger opening 54.

First end 48 of hanger 40 is positioned such that hanger opening 54aligns with duct opening 52 of intermediate duct 26B. Fastener 44 isinserted into openings 52 and 54 such that head 60 rests on drop plate68 located in opening 62 of hanger 40. Drop plate 68 also includeswebbing 69, which strategically increases the width of drop plate 68.One or more webbings are placed around drop plate 68 to prevent fastener44 from dropping through opening 62 during rotation or if it becomescanted in openings 52 and 54 during installation. For properinstallation of suspension system 30, head 60 is rotated approximatelyninety degrees from the orientation in which it passed through openings52 and 54 such that its width is substantially transverse to the widthof openings 52 and 54. Thus, fastener 44 is restrained from pulling outof openings 52 and 54 by flanges 56.

Hanger 40 also includes web 70 for impeding rotation of head 60 offastener 44. Web 70 comprises a flange or tab that extends into opening54 from flange 56 such that it engagable with head 60 when head 60 isrotated in opening 62. During assembly of suspension system 30, nut 46is torqued onto shaft 58 thereby causing clockwise (as viewed from theexterior of duct 26B) rotation of head 60. Web 70 pushes back on head 60thereby holding head 60 substantially perpendicular to opening 54 whilesuspension system 30 is being assembled. A second web is included onhanger 40 positioned one hundred eighty degrees from web 70 on opening54 such that it assists web 70 in restraining rotation of head 60.

Likewise, in order to prevent head 60 from spinning during disassemblyof suspension system 30, bushing 42 is provided with tab 66. Tab 66impedes free rotation of head 60 in opening 62 in the counterclockwisedirection. Tab 66 transmits torque during disassembly of suspensionsystem 30 by substantially preventing rotation of head 60. Onealternative to use of tab 66 is to rely on web 70 to preventcounterclockwise rotation of head 60. Often times, however, the torquerequired to dislodge nut 46 from shaft 58 exceeds the torque necessaryto fully seat nut 46 during installation, the torque for which web 70 isdesigned to withstand. This is typically due to residual heat stressesimparted in fastener 44, dirt, corrosion or the like. The torquerequired to remove nut 46 could damage hanger 40 before head 60 wouldeven be allowed to rotate in the counterclockwise direction until itcontacted web 70. Thus, for weight and space considerations, web 70 istypically designed to withstand only the necessary assembly torqueimparted in fastener 44. Another alternative to use of tab 66 is toinclude additional webs on opening 54 to prevent counterclockwiserotation of head 60. This alternative, however, could interfere with thecapability of head 60 to be rotated perpendicular to opening 54 (e.g.prevent head 60 from engaging flange 56) after being inserted intoopening 54. Therefore, tab 66 is included in bushing 42 to transmitdisassembly torque to duct 26B, which can absorb much larger stressesthan hanger 40.

Tab 66 also ensures that head 60 is engaged with web 70 duringinstallation to prevent improper alignment of head 60. Bushing 42 isseated in opening 52 of duct 26B and includes flange 72 around its outerperiphery. Flange 72 prevents bushing 42 from passing through and intoduct 26B and also assists in sealing opening 52. Tab 66 prevents bushing42 from being inserted into opening 52 unless head 60 is rotatedsubstantially transverse to openings 52 and 54 and flush with web 70. Itis preferable that head 60 is transverse to openings 52 and 54 so thatmaximum tension will be transmitted from duct 26B to hanger 40 throughshaft 58.

The sizing and positioning of tab 66 is such that bushing 42 can beinserted into opening 52 in two positions; with tab 66 on either theupstream or downstream side of opening 52. The exact sizing anddimensions of bushing 42 and tab 66 depends on the specific designparameters of suspension system 30 and aircraft 10. When bushing 42 isinserted into opening 52, tab 66 extends into opening 54 such that itswidth is substantially transverse to the widths of openings 52 and 54.The width of tab 66 is then substantially aligned with and abuttedagainst the width of head 60, ensuring that head 60 is properlytransverse with opening 54 when suspension system 30 is assembled. Also,the length of tab 66 is aligned with and abutted against the side of theweb opposite web 70 (not seen in FIG. 5).

The length of tab 66, i.e. the distance it extends into opening 62 ofhanger 40 depends on the position of drop plate 68 and the thickness t(FIG. 4) of head 60. In the various embodiments of the presentinvention, tab 66 extends to within close proximity of drop plate 68such that head 60 is prevented from rotating by tab 66. In other words,the distance between the bottom of tab 66 and the top of drop plate 68,when assembled, is less than thickness t of head 60.

FIGS. 6A-6C show perspective, side and bottom views of bushing 42,respectively. Bushing 42 can be comprised of any high strength, heatresistant material, such as stainless steel or titanium, that iscompatible with other materials used in suspension system 30.Specifically, bushing 42 is comprised of a material having a coefficientof thermal expansion compatible with the coefficients of thermalexpansion of hanger 40, fastener 44 and nut 46. For example, afterundergoing thermal expansion consistent with temperatures reached inengine 18, opening 52 remains sealed with duct 26B, yet does not causeloss of preload in fastener 44, duct 26B or hanger 40.

FIG. 6B shows a bottom view of bushing 42. Bushing 42 includes bushingopening 64, flange 72, depression 74 and tab 66. The precise dimensionsof bushing 42 depend on the specific engine and aircraft combination andthe forces bushing 42 is required to transmit.

Opening 64 is generally positioned at the center of the main body ofbushing 42 and has a diameter that approximates the diameter of shaft 58of fastener 44 such that a seal is formed. Depression 74 surroundsopening 64 and extends into the underside of bushing 42, primarily as aweight reduction means. The profile of the main body of bushing 42approximates the profile of opening 52 in duct 26B such that the mainbody assist in transmitting torque from tab 66 to duct 26B. The body ofbushing 42 also matches with opening 52 such that bushing 42 will retainshaft 58 of fastener 44 aligned in opening 64.

Flange 72 extends around the periphery of bushing 42 such that bushing42 cannot pass through duct opening 52. Flange 72 has width X suchbushing 42 forms a seal with opening 52 of duct 26B. Flange 72 alsoprovides nut 46 with a ledge for transmitting tensile forces fromfastener 44 to duct 26B.

Tab 66 has a “D” shaped cross-section, but in other embodiments can haveother shapes. Tab 66 includes generally planar face 76 for aligning withthe width of head 60 of fastener 44. Face 76 is generally planar suchthat it fully engages head 60 and evenly transmits torque to duct 26B.

FIG. 6C shows a side profile of bushing 42, in which the main body ofbushing 42 is shown having depth D, and tab 66 is shown having thicknessT. In one embodiment, depth D of the main body of bushing 42 is sized tomatch the thickness of duct 26B such that head 60 slightly compressesbushing 42 when suspension system 30 is assembled.

Tab 66 functions to transfer torque from head 60 to duct 26B. ThicknessT of bushing 44 can be increased to absorb higher torque loads for usewith, for example, heavier duct liners. In other embodiments of theinvention, bushing 42 includes additional features, such as a secondtab, for further improving the torque transfer capabilities of bushing42.

FIG. 7 shows a second embodiment of bushing 42 in which second torquetab 78 is added. Torque tab 78 allows torque to be transmitted from head60 to duct 26B from another source such that heavier forces can be usedfor assembling heavier suspension systems. [Insert text]

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A bushing for assembling an exhaust duct liner with an exhaust ductin a gas turbine engine, the bushing comprising: a body insertable intoa duct opening in an exhaust duct; a bushing opening extending throughthe body for receiving a shaft of a liner fastener; and a first tabprotruding from the body for extending into an interior of the duct toprevent rotation of a head of the liner fastener.
 2. The bushing ofclaim 1 wherein the bushing includes a peripheral flange extending alonga periphery of the body to prevent the body from passing through theduct opening.
 3. The bushing of claim 2 wherein the peripheral flangeextends around an entirety of the periphery of the body to form a seal.4. The busing of claim 2 wherein the body has a cross-sectional profilehaving a major axis and a minor axis in a plane perpendicular to an axisof the bushing opening.
 5. The bushing of claim 4 wherein the bushingopening comprises a circular opening positioned at the intersection ofthe major axis and the minor axis.
 6. The bushing of claim 5 wherein thebody has an oval cross-sectional profile.
 7. The bushing of claim 4wherein the tab is positioned on the major axis.
 8. The bushing of claim7 wherein the tab includes a flat surface extending in the direction ofthe minor axis.
 9. The bushing of claim 7 wherein the body includes adepression adjacent the bushing opening to reduce weight.
 10. Thebushing of claim 9 and further comprising a second tab spaced along thebushing opening opposite the first tab.
 11. A bushing assembly formounting an exhaust duct, the assembly comprising: a fastener forconnecting with the exhaust duct, the fastener comprising: a shaft forextending through a duct opening in the exhaust duct; and a headconnected to an end of the shaft for positioning inside the exhaustduct; a bushing insertable into the duct opening, the bushingcomprising: a body for insertion into the duct opening, the body havinga major axis and a minor axis; a bushing opening extending through thebody to receive the shaft of the fastener; and a first tab protrudingfrom the body for extending into the duct, the first tab beingpositioned to prevent rotation of the elongate head within the duct; anda nut for engaging the shaft of the fastener and securing the fastenerwith the bushing.
 12. The bushing assembly of claim 11 wherein the firsttab is positioned such that a major axis of the fastener head istransverse to the major axis of the body when the head is adjacent thebody.
 13. The bushing assembly of claim 11 wherein the first tab ispositioned on the major axis.
 14. The bushing assembly of claim 11wherein the bushing opening and the shaft have matching profiles. 15.The bushing assembly of claim 11 wherein the bushing includes aperipheral flange extending along a periphery of the body to prevent thebody from passing through the duct opening.
 16. The bushing assembly ofclaim 11 wherein the bushing comprises a second tab spaced along thebushing opening opposite the first tab.
 17. The bushing assembly ofclaim 11 wherein the bushing and the fastener have similar coefficientsof thermal expansion such that the bushing maintains a seal around theshaft of the fastener.
 18. A bushing comprising: an oblong bodycomprising: an outer face; an inner face; and a side face extendingbetween the outer face and the inner face; a flange extending along aperiphery of the side face coplanar with the outer face; a fastener boreextending through the body from the outer face to the inner face; and atab extending perpendicularly from the inner face.
 19. The bushing ofclaim 19 wherein: the inner face has a pair of elongate side edgesextending between a pair of shorter side edges; the fastener boreextends through a center of the inner face; and the tab extends from theinner face between a shorter edge and the fastener bore.
 20. The bushingof claim 19 wherein the tab comprises a flat surface extendingperpendicular to the pair of elongate edges and facing the fastenerbore.